Folding wine-rack material expands when squeezed

Push this material, and it pushes back.

Take almost any solid material and squeeze it with an industrial piston. It will compress to a greater or lesser degree, depending on the kind of material and how hard you squeeze. However, certain unusual solids have the opposite response: they push back against compression, actually expanding in the direction they are squeezed. These materials are interesting in their own right, but also have potential applications in very sensitive pressure sensors, compensators in systems affected by humidity, or even artificial muscles.

In studying why some solids behave this way, Andrew B. Cairns and colleagues discovered both the secret to their properties and substance that shows the most extreme example of negative compressibility yet found. This material, a lattice of molecules containing gold and zinc, resembled a folding clothes rack on the microscopic scale. When squeezed, it pushed back more than double the amount normal materials typically compress. The researchers attributed this remarkable ability both to the springy internal structure and to the interactions between the gold atoms.

Most materials aren't very compressible. If you exert 10,000 times atmospheric pressure—roughly ten times the pressure at the bottom of the Mariana Trench—a typical metal or ceramic will only compress about 0.5 percent. (Kids! Try this at home!) While more squishable materials exist, these usually aren't very valuable for engineering purposes.

On the other hand, materials with negative compressibility expand when squeezed. Until the current study, only 13 such materials were known. The best of these expanded at a rate of about 0.12 percent for every 10,000 atmospheres of pressure exerted.

Compressibility

For those keeping careful track, the numbers I reference about percentage of compression or expansion are based on a simplified model, so they should be considered for comparison purposes only. The true compression and expansion amounts themselves depend on how much pressure is applied, so that materials "give" relatively less at higher pressures than at lower values.

No material can exhibit negative compressibility in all dimensions at once: if it expands in one direction, then it will contract in the two directions perpendicular to the expansion. The researchers' insight was to realize that they could maximize negative compressibility in one dimension by simultaneously maximizing positive compressibility in the other two. This kind of "wine rack" configuration (their words) is similar to an example found in nature: tendon structures in the legless amphibians known as caecilians.

The struts of the wine rack in the new experiment were linear molecules with a gold atom at the center, capped on each end with a pyramid of nitrogen and zinc atoms. (The full chemical formula is Zn[Au(CN)2]2, or zinc(II) dicyanoaurate(I). I'll stick to calling it a wine rack.) The gold atoms in the crystal structure formed a star-like pattern known as a kagome net, which is known to be very deformable—hence the wine rack analogy.

The total structure of the crystal lattice therefore lends itself to extremely compressible behavior in two dimensions, but correspondingly radical negative compressibility in the third. The researchers achieved greater than 3 percent expansion for 10,000 atmospheres of pressure. They noted that these extreme compressibility values don't hold up forever: they eventually resulted in a change in the crystal structure, as one might expect.

This wine rack material is transparent to light, so the authors suggested it could be used immediately in pressure sensors. Light shining through the material would experience changing refraction as the crystal expanded, which could be used to measure very precise changes in pressure. Similarly, since some biological systems possibly already exploit negative compressibility (though this isn't completely certain), similar materials could be used to fabricate artificial muscles or tendons, useful in prosthetics or other biomedical applications.

Well, I certainly didn't expect The Core to get bumped up a notch on movie plausibility today. Mind you, it still sits very low.

The only thing they got right in that movie was that people live on the Earth.

The background on that movie always amused me, because it was taken pretty much straight from the amazing PS1 action-rpg "The Granstream Saga." Last game I'm waiting to hit the PS1 classics section of the PS1 store (still have my disks, but it's nice not to have to worry about them, and easily play on the go on my Vita).

Also, to the guy above, I'm not sure that word means what you think it means.

Looks like one of the four basic structures of matter is slowly becoming not so basic. Then again that notion was really blown away as soon as subatomic particles were discovered.

Such is the nature of science. It's not the first time it's happened, nor will it be the last.

The example given in Isaac Asimov's amazing essay, "The Relativity of Wrong," about the evolution of theories from flat earth to the current pear-shaped gives a perfect example of this fundamental aspect of science and how we learn. What we may take for granted now, or think universal, may prove not to be quite as it seems. It could be drastically different, or it could be a subtlty, but the core holds. We're always learning, and we shouldn't ever be surprised (or disappointed) when we discover something new, even if it means we were wrong before (regardless of the degree of wrong).

It sounds like it's a matter of equilibrium of forces. As it's compressed the weak and strong forces (which lose influence with distance at an astronomical rate; at least I assume these are the forces they're talking about) gain influence, and so due to the molecular structure in some, it causes these to repel more than the compressing force that is applied.

Didn't have much luck finding graphics or videos to explain it, but i'm on my tablet, so it's not as easy to search video results or photos (connection here is really slow too).

I think that the description in the article is misleading. The article makes it sound as though if you push in one direction on the material, it expands *in that direction*. This seems impossible to me -- just look at the (single-axis) stress-strain relationship -- the slope is wrong and the material will be unstable. A simpler way to see the problem: get a piece of this material and put it on your desk. Put a weight on top of it. The material will magically become taller, pushing the weight up, which will add more force (by F=ma -- the weight is accelerating upwards), which will cause the material to get even taller, etc.

AFAICT what's going on is that, if you put the material under *isotropic* (same in all directions) pressure, the material will expand in one direction (and compress in another direction). So the total volume it takes up decreases, but it still gets bigger in one direction.

This is still really cool, but it's not as cool as the article makes it sound.

I think that the description in the article is misleading. The article makes it sound as though if you push in one direction on the material, it expands *in that direction*. This seems impossible to me -- just look at the (single-axis) stress-strain relationship -- the slope is wrong and the material will be unstable. A simpler way to see the problem: get a piece of this material and put it on your desk. Put a weight on top of it. The material will magically become taller, pushing the weight up, which will add more force (by F=ma -- the weight is accelerating upwards), which will cause the material to get even taller, etc.

AFAICT what's going on is that, if you put the material under *isotropic* (same in all directions) pressure, the material will expand in one direction (and compress in another direction). So the total volume it takes up decreases, but it still gets bigger in one direction.

This is still really cool, but it's not as cool as the article makes it sound.

This was my question exactly. I wish the article had been clearer on that point.

I think that the description in the article is misleading. The article makes it sound as though if you push in one direction on the material, it expands *in that direction*. This seems impossible to me -- just look at the (single-axis) stress-strain relationship -- the slope is wrong and the material will be unstable. A simpler way to see the problem: get a piece of this material and put it on your desk. Put a weight on top of it. The material will magically become taller, pushing the weight up, which will add more force (by F=ma -- the weight is accelerating upwards), which will cause the material to get even taller, etc.

AFAICT what's going on is that, if you put the material under *isotropic* (same in all directions) pressure, the material will expand in one direction (and compress in another direction). So the total volume it takes up decreases, but it still gets bigger in one direction.

This is still really cool, but it's not as cool as the article makes it sound.

No I think it really is saying that it expands in the direction you are applying stress. Otherwise, what you describe is pretty much how all normal materials behave.

As per the story " they push back against compression, actually expanding in the direction they are squeezed"

It sounds like it's a matter of equilibrium of forces. As it's compressed the weak and strong forces (which lose influence with distance at an astronomical rate; at least I assume these are the forces they're talking about) gain influence.

That's a very helpful link. Check out the diagram where it clearly shows the object shrinking along the axis of tension -- and thus presumably it would similarly expand along the axis of pressure, which would double make sense because otherwise this isn't a "meta" material at all but just a normal one.

I do not think they mean the weak nuclear force when they say "The two inner most particles intentionally have a weak force. " I believe they mean a weak as in measly bond, and the breaking of this bond appears to be a key part of the process. If they were actually getting atoms close enough together that the weak force was relevant then there'd be some rather undesirable reactions going on.

Cool. I'll bet the military and law enforcement could turn this into armor somehow. If it expands in the direction of force, then when a bullet hits it, it may be able to "recoil" back so that the bullet simply bounces off. How cool would that be. It'd be like the armor used in the Iron Man suit.

The weight doesn't increase enough to provide a perpetual "feedback loop" in which the weight constantly increases. There's probably some loss of energy somewhere, maybe exothermically due to interatomic friction. So the initial energy eventually gets "worn out".

Shavano wrote:

mkuch90 wrote:

Well, I certainly didn't expect The Core to get bumped up a notch on movie plausibility today. Mind you, it still sits very low.

I find something quite troubling about this. Make a block of this material. Set a weight on it. Since the block has negative compressibility, it raises the weight, which does work.

It sounds like it's a matter of equilibrium of forces. As it's compressed the weak and strong forces (which lose influence with distance at an astronomical rate; at least I assume these are the forces they're talking about) gain influence.

That's a very helpful link. Check out the diagram where it clearly shows the object shrinking along the axis of tension -- and thus presumably it would similarly expand along the axis of pressure, which would double make sense because otherwise this isn't a "meta" material at all but just a normal one.

I do not think they mean the weak nuclear force when they say "The two inner most particles intentionally have a weak force. " I believe they mean a weak as in measly bond, and the breaking of this bond appears to be a key part of the process. If they were actually getting atoms close enough together that the weak force was relevant then there'd be some rather undesirable reactions going on.

That makes more sense (though the other option didn't seem out of the question when we were talking about absurdly high pressures). In my defence, I was posting from my tablet while at the pub at the time haha. This also isn't my particular area of expertise; been over a decade (high school) since I did anything directly relating to materials sciences.

One correction though. I think that the article mentions there are only 13 canonically known materials with negative linear compressibility (NLC) (i.e. as of 1998). These systems had a compressibility at best of 0.12 % / 10,000 atm of pressure.

However, it goes on to state that since 1998 there have been four systems with NLC discovered (in addition to the system presented in the article). Each of the recent systems has a larger NLC than the canonical compounds (up to 10x), but still none that rival the material in the current study (about 400x).